This invention relates to normally tacky and pressure-sensitive adhesive tape, especially tape products having excellent shear and adhesion properties. Preferred embodiments of the invention include general purpose masking tapes and adhesive transfer tapes.
For over half a century normally tacky and pressure-sensitive adhesive tape has been widely used throughout the world for a variety of masking, holding, binding, protecting, sealing, marking, and other purposes. In its simplest form, such tape comprises a sheet backing to one face of which is adhered a coating of normally tacky and pressure-sensitive adhesive, a composition which possesses a four-fold balance of adhesion, cohesion, stretchiness and elasticity. This balance is achieved when the adhesive possesses a suitable combination of both viscous and elastic properties. Pressure-sensitive adhesives are discussed extensively in Chapter 17 of Houwink and Salomon, Adhesion and Adhesives, Vol. 2, Elsevier Publishing Company, Amsterdam, Netherlands, 1967.
General purpose pressure-sensitive adhesive tapes have almost always employed a rubber-resin functional adhesive, which may be coated from solution or emulsion, applied as a hot melt, or polymerized in situ. For applications where optical clarity or adhesive bond longevity is required, acrylic or silicone pressure-sensitive adhesives are used, their relatively higher cost precluding general use.
In searching for an alternative to rubber-resin for general purpose pressure-sensitive adhesives, a great deal of attention has been paid to polyurethane chemistry, and many patents on nominally pressure-sensitive polyurethane-based adhesives have issued over the last quarter century. Polyurethanes can be readily formed from raw materials which are themselves comparatively inexpensive; combinations of diols and triols can be reacted with diisocyanates to obtain a wide spectrum of elastomeric products having excellent internal strength and heat resistance, as well as some of the rheological properties of pressure-sensitive adhesives. Since the present cost of polyurethane-based adhesives is approximately half that of either rubber-resin or acrylic adhesives, it would be expected that they would be in widespread use today. Such, however, is not the case, for tapes utilizing polyurethane-based adhesives have been found suitable for only a few specialized uses.
A pressure-sensitive adhesive must possess some degree of elastic compliance and viscous flow, enabling it to relieve the stresses to which it is subjected to in use; see "Pressure-Sensitive Adhesives" in "Treatise on Adhesion and Adhesives", Vol. 2, "Materials," R. I. Patrick, Ed., Marcel Dekker, Inc. New York, 1969 and Chapter 1 in "Viscoelastic Properties of Polymers", John D. Ferry, 3.sup.d Edition, John Wiley & Sons, New York, N.Y., 1980.
When tested in shear, a true pressure-sensitive adhesive exhibits both elastic (recoverable) deformation and viscous (non-recoverable) deformation. These materials can be described mathematically from compliance data generated from shear creep experiments as .gamma..sub..tau. =.sigma..sub.o J.sub.e.sup.o +.sigma..sub.o t/.eta. (where t=time, J.sub.e.sup.o =elastic
deformation, .sigma..sub.o =stress, .eta.=viscosity, and .gamma.=strain. In this mathematical description, the term .sigma..sub.o J.sub.e.sup.o reflects the elastic or the recoverable energy of deformation in a shear mode, while the term .sigma..sub.o .tau./.eta. reflects the viscous component, or non-recoverable energy. Typically the viscous component represents about 25% of non-recoverable deformation, although this value is not limiting in terms of the non-recoverable deformation of pressure-sensitive adhesives.
Polyurethane-based adhesives, however, do not seem to possess these same characteristics, functioning either like weak rubber bands or simply as high viscosity liquids. The former adhesives possess no viscous component but exhibit both elastic and delayed elastic deformation. They continue to elongate when subjected to a stretching force but return to essentially their original dimensions after the stretching force is removed; i.e., they are elastic. The less the degree of crosslinking, the greater the delayed elastic response. When such a soft polyurethane is stressed, shear deformation can continue to occur over an extended period, which may give the appearance--but not have the actual physical characteristics--of viscous flow. Thus, these materials possess no viscous component and are mathematically described by .gamma..sub..tau. =.sigma..sub.o J.sub.e, in which case J.sub.e contains the components of both elastic and delayed elastic deformation. Tapes made with such adhesives are deficient for general purpose use, since they tend to fail by gradually peeling away from surfaces to which they have been applied.
High viscosity polyurethanes are typically obtained by using a substantial excess of polyol. When viscous flow is attained in this manner, the adhesive at least partially transfers to the surface to which it is applied, leaving a deposit upon removal, and its cohesive strength is too low to withstand the stresses applied in normal uses. These materials are also mathematically described by the equation .gamma..sub..tau. =.sigma..sub.o J.sub.e.sup.o +.tau..sub.o t/.sub..eta., but in this case, the second term, .sigma..sub.o t/.sub..eta., dominates the equation.
It is primarily for the rheological reasons just discussed that, in spite of their attractive cost, polyurethane "pressure-sensitive" adhesives have not enjoyed much success in the marketplace. It has not been possible to achieve the requisite balance between viscosity and elasticity with polyurethane chemistry; the resulting adhesives are unacceptable for most pressure-sensitive adhesive applications, even though they possess rheological properties found in many pressure-sensitive adhesives.
It is obvious from the preceding discussion that it is possible to describe and distinguish among similar or different viscoelastic materials. From shear creep measurements, the compliance, J, is directly obtained. Thus ##EQU1## Note that a plot of the above equation yields a straight line having a slope of 1/.sub..eta. (the viscous component) and an intercept of J.sub.e.sup.o (the elastic component).
Over the years, some tape or tape-like products have been made (usually for very specific end uses) incorporating two or more consecutively applied layers of adhesive. For example, note the following U.S. Pat. Nos.: 997,125, which shows a fabric coated with two non-tacky adhesive layers, the outer layer having a lower melting point than the inner; U.S. Pat. No. 2,652,351, which describes a tape having two coatings of apparently identical pressure-sensitive adhesive; U.S. Pat. No. 3,161,533, which discloses a pressure-sensitive adhesive tape in which two silicone pressure-sensitive adhesives, differing in filler content, are consecutively coated on a backing; U.S. Pat. No. 3,340,088, which teaches the consecutive coating of a thin, tacky rubber-resin primer and a low-tack rubber-resin pressure-sensitive adhesive; U.S. Pat. Nos. 3,811,438 and 3,885,559, which describes adhesive tapes in which the backing is first completely coated with one pressure-sensitive adhesive and then strip-coated with a somewhat more aggressive pressure-sensitive adhesive; U.S. Pat. No. 3,812,005, which discloses a base sheet consecutively coated with an adhesive and a "self-sticking layer".
U.S. Pat. Nos. 4,112,177 and 4,260,659 disclose tape in which one pressure-sensitive adhesive coating is overcoated with a second pressure-sensitive adhesive coating. In each case, the first pressure-sensitive adhesive possesses an essential property in which the second adhesive is deficient. In each case, this essential property (respectively shear and porosity) is much lower in the final composite construction than it is in the first coat, reflecting a compromise between the properties of the two adhesive systems. In both cases the overcoated top adhesive must be more compliant than the bottom coat to achieve desired performance levels.
Much of the preceding discussion, mutatis mutandis, is applicable to adhesive transfer tapes. In such products the backing on which the adhesive is coated is a release liner; in typical use, the exposed adhesive surface is placed in contact with a first substrate to which it is to be permanently bonded, after which the release liner is stripped away and a second substrate permanently bonded to the newly exposed adhesive surface.